This invention relates to methods of improving the steering performance of vehicles, particularly passenger vehicles having pneumatic tires.
The term xe2x80x9csteering performancexe2x80x9d (or simply xe2x80x9csteeringxe2x80x9d) refers to a vehicle driver""s feeling that a vehicle""s steering (and/or xe2x80x9chandlingxe2x80x9d) is responsive to movement of the steering wheel. The better the steering performance, the better the driver""s xe2x80x9cfeelingxe2x80x9d of having control over the vehicle""s steering. Because it relates to a xe2x80x9cfeelingxe2x80x9d on the part of a driver, steering performance is essentially a subjective evaluation of a vehicle""s steering. Steering performance can change over time, mainly deteriorating as components in the vehicle steering system wear, age, or suffer damage. Steering system components include the steering wheel, the tires and wheels, and everything in between such as the steering box, any power assist components, and linkages and joints. Steering performance can also vary with operating conditions, including, for example, road texture, vehicle speed, steering wheel settings, minor tire inflation pressure changes, and tire/wheel uniformity changes (e.g., balance).
Worsening steering performance is generically referred to as xe2x80x9csteering performance lossxe2x80x9d (SP-Loss), or xe2x80x9cgive-upxe2x80x9d. A steering system or component which is resistant to steering performance loss is xe2x80x9crobustxe2x80x9d or can be said to possess xe2x80x9csteering performance robustnessxe2x80x9d. Similarly, any component change which appears to delay or prevent SP-Loss in a vehicle steering system can be said to improve steering performance robustness. Also, it has to be mentioned that SP-Loss is more noticeable on vehicles which have, in general, a very good and crisp steering. Finally, it has been noted that steering component changes which improve steering performance robustness usually also enhance the driver""s perception of steering performance. The inverse may not be true, i.e., a component change which enhances steering performance initially may not be robust and therefore quickly deteriorates to yield a net steering performance loss.
Steering performance loss is mostly a concern in passenger vehicles with pneumatic tires and power assisted steering (power steering), although the phenomenon has also been observed in passenger vehicles without power steering. Although a trained driver can determine steering performance at virtually any vehicle speed, the steering performance (and therefore a change in performance, e.g., SP-Loss) is most noticeable above a certain vehicle speed threshold. Even though SP-Loss is generally a change over time, it can be practically instantaneous.
Certain vehicles appear to be more susceptible to SP-Loss, and it has been noted (especially on these vehicles) that steering performance is affected by differences in tire construction, or even by changes from one tire to another of the same tire construction. (The common industry term xe2x80x9ctire constructionxe2x80x9d includes all elements of a tire""s designxe2x80x94including, for example, tire/carcass shape, tread pattern, number and type of plies, materials and manufacturing methods used, etc.) It is well known that tire uniformity (e.g., balance) varies from tire to tire, and that an unbalanced tire causes vibrations which are felt in the steering, therefore tire uniformity is almost universally controlled in tire and wheel design, during tire and wheel manufacturing, and after forming a tire/wheel assembly. It is generally assumed that improved tire/wheel uniformity will improve steering performance and hopefully will also help with steering performance robustness. As noted above, this is not always the case, and thus a great deal of research has been directed toward additional solutions to SP-Loss, such as various tire design changes.
Regardless of tire construction/design, the tire and vehicle industry generally strives for the best possible tire uniformity and, by extension, uniformity of the tire/wheel assembly whenever a tire is mounted on a wheel for use on a vehicle. This is a multi-part optimization process whereby the tire manufacturer strives for optimum tire uniformity, the wheel manufacturer strives for optimum wheel uniformity, and then the vehicle operator has the tire/wheel assembly tested and corrected for xe2x80x9cbalancexe2x80x9d.
Tire uniformity and tire/wheel balance are well known topics in the tire industry. A brief description of certain relevant portions of these topics will now be presented.
Uniformity and Balance
Tire manufacturers generally perform quality checks on tires at various points during the manufacturing process. Tire uniformity is an important, performance-related check typically performed on a tire uniformity machine (TUM) which is well-known in the art and will not be described in detail herein. Tire uniformity machines most commonly rotate a tire mounted on a known-to-be-uniform or xe2x80x9ctrue runningxe2x80x9d wheel, and measure variations in forces on the wheel axis (or on a load wheel) and/or measure variations in tire outer surface positions. Typical force variation measurements include radial force variation (RFV) which is indicative, for example, of static imbalance or radial runout; and lateral force variation (LFV) which is indicative, for example, of couple imbalance, lateral runout, or tire radial runout skewness. Tire surface measurements are directly indicative of runout conditions and conicity. Another measurement, generally done on a sampling basis on special laboratory grade, high speed TUMs, is tangential force variation (TFV), or fore-aft force variation which is experienced at the surface of contact between a tire and a road surface in a direction both tangential to the tire tread and perpendicular to the tire axis of rotation.
In terms of effect on a vehicle and its tires, all of the types of force variations can cause vibrations dependent upon the magnitude of the force variation (modified by vehicle characteristics such as wheel suspension mass/stiffness/damping conditions). The lateral force variation (and/or couple imbalance) primarily cause vibrations due to a wobbling motion of the tire, with the axis of rotation for the oscillation being vertical or horizontal, parallel to the tire""s circumferential plane, and approximately centered within the tire/wheel volume. In contrast, radial and tangential force variation and/or static imbalance mainly cause vibrations due to movement in vertical and fore-aft directions (although some lateral movements exist, they are distributed symmetrically about the equatorial plane and involve only a small percentage of the total tire/wheel assembly mass).
Static and Couple Imbalances
Generally speaking, when a tire/wheel assembly is xe2x80x9cbalancedxe2x80x9d, the modern practice is to test, and correct if necessary, both the static and the couple balance of the assembly. This balancing is generally performed using special-purpose equipment. For couple balancing, the equipment generally rotates the tire/wheel assembly at a relatively high speed, and the tire is not in contact with any surface (compare to the road-wheel used in TUM testing).
Static imbalance arises in a rotational system such as a tire and wheel assembly when the mass of the rotating tire/wheel assembly is non-uniformly distributed about the axis of rotation in such a way that the sum of the centrifugal force vectors arising from each moving part of the rotating system is non-zero. The term xe2x80x9cstatic,xe2x80x9d when used in reference to rotational balance, refers to the fact that rotational motion is not needed to identify, locate and correct the rotational imbalance. That is, a wheel that has a static imbalance will, at certain stationary angular orientations about the horizontal axis of rotation, exert a torque vector about the axis of rotation, due to gravity forces. An optimally balanced tire/wheel system will produce no such torque vector about the axis of rotation. Of course, it must be acknowledged that no rotational system can have xe2x80x9cperfectxe2x80x9d static balance, but that adequate or optimal static balance can be achieved in real-world rotational systems such as tire/wheel assemblies and aircraft propellers and the high-rotational-speed components of gas turbine and steam turbine engines. One precise way to describe and define ideal static balance is to say that a rotating system is in static balance if all of the centrifugal force vectors (which act perpendicularly to the axis of rotation) have a sum that is zero.
In contrast to static imbalance, couple imbalance can, for all practical purposes, be detected only during rotational motion and therefore requires dynamic balancing machines. That is, a rotational system such as a tire/wheel assembly can appear to have perfect static balance, and yet, during rotation, vibrations associated with imbalance forces will arise, due to couple imbalance.
A dynamic balancing machine can be used to detect and correct both couple imbalance and static imbalance, and therefore a tire/wheel assembly that is characterized as xe2x80x9cdynamically balancedxe2x80x9d is generally understood to be both statically and couple balanced. The definition given above for static balancexe2x80x94namely, that all of the centrifugal force vectors have a sum that is zeroxe2x80x94can be supplemented to provide a corresponding definition of dynamic balance: i.e., dynamic balance of a rotational system exists when the sum of all the centrifugal force vectors is zero (static balance), and the sum of the moments of these centrifugal force vectors about any axis that is perpendicular to the axis of rotation is zero (couple balance).
One example of a statically balanced but dynamically unbalanced tire/wheel assembly would be one in which one sidewall of a tire is non-uniform in such a way that some angular portion of the sidewall is either lighter or heavier than the other portions of the sidewall while, at the same time, the other sidewall has exactly the same mass-distribution properties but is oriented about the axis of rotation in such a way that the respective non-uniformities of each sidewall are oriented angularly apart from one another with respect to the tire/wheel""s axis of rotation. In such a tire/wheel assembly, the centrifugal force vectors would thus have a non-zero resulting moment and would tend to rotate the tire about an axis that is perpendicular to both the axis of rotation and the direction of these centrifugal force vectors. However, with regard to static balance, the centrifugal force vectors for such a tire as a whole could have a sum that is zero if the locations of excess mass associated with the respective sidewalls in the above example are located angularly apart from one another. For example, if two equal excess masses xe2x80x9cMxe2x80x9d are located one at a point in each sidewall, then dynamic imbalance with static balance (i.e. pure couple imbalance) will occur if mass M in the first sidewall is located 180 degrees around from mass M in the second sidewall, and both masses M are located at the same radius. (In the irregular sidewall example given above for dynamic imbalance, if only one of the sidewalls is unbalanced, the tire/wheel assembly will have both a static and a couple imbalance.)
Other Non-Uniformities
In addition to static and couple mass imbalances, rotational vibrations can arise from other non-uniformities in tire/wheel assemblies. For example, a tire might have a tread or sidewall(s) that has greater or lesser flexibility (stiffness) within one angular portion compared to other portions of the tread or sidewalls. Such a tire/wheel assembly might have xe2x80x9cperfect,xe2x80x9d i.e., as close to perfect as is practical, couple and static balance, but when it is operated upon a vehicle, the portion of the tread or sidewall that is either softer or stiffer will interact with the road surface in ways that will give rise to vibrations comparable to (similar in some regards, but somewhat more complex than) those of a tire/wheel assembly that is unbalanced.
For example, if a portion of the tread is more or less flexible than the other portions of the tread, and the tire is otherwise uniform in its properties across its lateral dimension, a resulting vibration may be comparable to that arising from a static imbalance. Or, if the stiffness properties of the tire are not uniform from side to side and along the tire circumference, then the resultant rotational vibrations might mimic the effects of a couple imbalance.
Similarly, tires with radial or lateral runout will interact with the road surface in ways that will give rise to vibrations comparable to those of a tire/wheel assembly that is unbalanced, statically or dynamically.
It is an object of the present invention to provide methods of improving vehicle steering performance and steering performance robustness (reducing the risk of steering performance loss), as defined in one or more of the appended claims.
According to the invention, a method of improving steering performance robustness in a vehicle having a plurality of tire/wheel assemblies, including a number of front tire/wheel assemblies and a number of rear tire/wheel assemblies, each tire/wheel assembly comprising a tire mounted on a wheel, comprises imparting a controlled amount of dimensional non-uniformity to at least one of the tire/wheel assemblies, preferably one or more of the front tire/wheel assemblies. Preferably all of the plurality of tire/wheel assemblies are statically and dynamically balanced.
According to an embodiment of the inventive method, the dimensional non-uniformity is imparted to the tire of the at least one tire/wheel assembly, preferably distributed meridionally symmetrically about the equatorial plane of the tire, and then the tire with the dimensional non-uniformity is corrected as needed to assure minimum lateral force variation. The dimensional non-uniformity can be imparted by imparting a different thickness to at least one tire component in at least one sector of the tire, thereby adding mass non-uniformity to the dimensional non-uniformity for a synergistic beneficial effect. Alternatively, the dimensional non-uniformity is imparted by manufacturing the tire such that the tire""s ply cords vary in length around the circumference of the tire, using a tire manufacturing method which includes, for example, inclining bead setters in the tire mold, preferably by approximately 0.5 degrees to approximately 3 degrees. Alternatively, the dimensional non-uniformity is imparted by varying a tread pattern for the tire. Alternatively, the dimensional non-uniformity is radial runout of the tire comprising at least one radial runout peak.
According to an embodiment of the inventive method, the dimensional non-uniformity in the at least one tire/wheel assembly is imparted as at least one radial runout peak, preferably approximately 0.1 mm to approximately 0.8 mm.
According to an embodiment of the inventive method, the dimensional non-uniformity is imparted by placing one or more shims between the wheel""s bead seats and the tire""s beads in the at least one tire/wheel assembly.
According to an embodiment of the inventive method, the dimensional non-uniformity in the at least one tire/wheel assembly is imparted to the wheel of the at least one tire/wheel assembly, preferably symmetrically about the equatorial plane of the wheel, and the wheel with the dimensional non-uniformity is corrected as needed to assure minimum lateral force variation in the tire/wheel assembly utilizing the wheel. For example, the dimensional non-uniformity can be an eccentric wheel, or can be radial runout of the wheel comprising at least one radial runout peak.
According to the invention, a method of improving steering performance robustness in a vehicle having a plurality of tire/wheel assemblies, each tire/wheel assembly comprising a tire mounted on a wheel, comprises minimizing lateral force variations, wobbling moment variations, couple imbalance, lateral run-out, tread snaking and radial run-out skewness, for all of the plurality of tire/wheel assemblies. Preferably, all of the plurality of tire/wheel assemblies are statically and dynamically balanced.
Other objects, features and advantages of the invention will become apparent in light of the following description thereof.